The use of rubber articles has always been subject to the problem of oxygen and ozone induced degradation. Over the years numerous solutions have been proposed to lessen or eliminate the destruction of the rubbery polymers by attack from ozone and/or oxygen, including the use of chemical antidegradants to prevent or lessen the degradation of polymers subject to attack by oxygen and ozone. More recently, several new approaches have been submitted to further enhance a rubber's ability to resist ozone and oxygen degradation. These new approaches include higher molecular weight materials, polymer bound materials and specific blends of antioxidants and/or antiozonants with waxes, rosins and the like. One problem that the prior art approaches have failed to solve is that the antiozonant at the rubber article surface is depleted after several years of service. This is especially true of articles such as truck tires that may be recapped or retreaded three or more times. In such use the rubber, especially the tire sidewall, must maintain extended antiozonant protection so that the tire carcass may be used to its fullest potential.
In general, the approach of this invention is to place the antidegradant in a capsule with specific wall to core ratios wherein the capsule has a diameter of 50 microns or less. The use of a microencapsulated antidegradant provides or releases additional antidegradant to the rubber compound to compensate for or replace the depleted or consumed antidegradant. Through the use of encapsulated antidegradants, the effective service life of a rubber article is greatly enhanced.
A microcapsule is a particle or droplet of one material encased in a protective wall of a different material: however, the core material may also be dispersed throughout the wall polymer. The resulting particle or droplet interacts with its environment differently than the noncoated particle or droplet ordinarily does. Generally, microcapsules range in size from about 3 millimeters down to a few micrometers (microns) or even less. As used herein, the term microcapsule means a capsule that may have one or several nuclei; that is, one or several droplets or particles may be incorporated into a single microcapsule. The capsule core may be liquid, solid, a solid suspended in liquid or one liquid dispersed in a second liquid. One very common application of microcapsule technology is in carbonless paper.
One benefit of encapsulation is that it can facilitate handling. For example, an encapsulated liquid may be handled much like a free flowing powder. Encapsulation provides for controlled release of the core material which may actually reduce the amount of active ingredient needed to do the job. With certain pharmaceuticals, encapsulation has been used to sustain a minimum effective dose level for extended periods of time: i.e., Contac.TM. cold medication.
Through extensive research and effort, the inventors herein have developed an encapsulation method, raw materials and other parameters that provides superior protection to rubber articles from oxygen and ozone attack.
Over the past 25 years a wide variety of methods and processes for encapsulating materials have been developed. Some of these, such as the fluidized bed process and spray drying, were adapted from equipment and processes already in use for other purposes. Others, such as phase separation and coacervation, were developed specifically for microencapsulation. These varied processes provide different techniques for producing capsules of varying sizes, alternative materials for the composition of the capsule cell or wall and various functional materials within the capsule. Some of these various processes are shown in U.S. Pat. Nos. 3,516,846, 3,516,941, 3,778,383, 4,087,376 and 4,251,386. It is known that a wide variety of different materials may be used in making the capsule walls including gelatin and synthetic polymeric materials. A variety of capsule forming materials are disclosed, for example, in U.S. Pat. Nos. 3,516,846, and 4,087,376.
The microencapsulated antidegradant of this invention is prepared by a spray drying technique. Typically spray dried microcapsules range in size from 3 to 80 microns. These microcapsules can be mononuclear or polynuclear. The wall material is typically 25 to 80% or more of the total product weight.
Conventionally, in the spray drying technique, the material to be encapsulated and the coating material are fed into the top of the drying chamber through an atomizing device. A high volume, high velocity flow of air is fed in and around the atomizer. The solvent is flash evaporated from the small droplets or with hot melt systems, the droplets are rapidly frozen. The air stream with the dry or hardened microcapsules passes into a cyclone separator where the product is separated from the exhaust air stream and collected. In the spray drying technique, the use of particular solvents, polymer concentrations, droplet size, solution viscosities and other variables are important considerations in the production of a suitable microcapsule.
The Applicants have discovered that the preparation of a microencapsulated antiozonant as described and claimed herein will survive Banbury mixing at temperatures in excess of 110.degree. C. and survive the curing process which achieves temperatures in excess of 150.degree. C. Further, it was unexpectedly observed that the microcapsules prepared according to this invention were compatible with rubber formulations, and did not interfere with the curing or vulcanization of the rubber and did not adversely impact the physical properties of the rubber vulcanizate. In addition, it was discovered that encapsulation of the antiozonant, in accordance with this invention, affected the rate at which the antiozonant became available for consumption at the surface of the rubber article and therefore greatly extends the effective lifetime of the rubber article. The prior art does not suggest, disclose, or contemplate the advancement to the state of the art as recited and claimed herein.